Cost analysis of glass types

[cheapo]

Du Pont Sentry

My mentioning laminated glass in my last post made me want to google it for solar panels and

DuPont (whudda they know next to kitchen geniuses like us) recommends their laminated glass for solar panels.

http://www2.dupont.com/SafetyGlass/en_US/whats_new/dupont_announces_35_mil.html

What appealed to me was its availability in thin lightweight sheets because glass is the big heavy in an aluminum framed plastic backed panel. They brag about how stiff it is, but it may still be too flexible for delicate cells and they may crack. Or it may be great.

It is probably pricey, but so is ordinary tempered.

[MikeSolar]

Standard tempered glass is around 80-82% transmissive, Low iron is around 92-95% transmissive. I buy a 4x8 sheet of low iron AFG glass (it has a new name now but I cannot remember what it is) for about $150. The glass is 3.2mm or 4mm standard thickness for both PV and thermal panels. 1/4" glass is not necessary. you do need to buy it in some quantity though (1 skid).

[longwolf]

...................
My point is that if one bonds the cells to the glass with plastic resin (Sylgard 184 or whatever), you eliminate 2 glass/air surfaces, and should pick up 8%, except where you have bubbles, but such should be minor.

Yep, I was surprised to learn that one while I was doing experiments with Clear Seal.
It's another great reason to encapsulate your cells!

...................
Although I agree with and respect the conclusions of longwolf, I would suggest taking both volt and amp readings off a single cell behind various glasses and bonded and unbonded with resin. Not to mention encased in EVA and the other options.

Readers may well consider such to be my job, and if I ever "get it together" (currently in process of much home remodeling) i hope to do my part.

I've been asking for results from a test like that.
There is a problem with doing the tests with single cells. There can be a big difference between the output of any two cells. Once you've hooked a bunch together the differences tend to average out. So you'ld have to pick your cells very carefully.
If you try it please post what you learn.

[cheapo]

"Reflections" on problems

Milosolar, it is good to know of alternatives, and that thin glass can be strong if tempered. My interest in 1/4 inch is that it tends to be code for certain doors and things and so is available used and cheap. Thinner is clearly lighter.

Longwolf, glad your meter readings confirmed my theory.

People seem to get obsessed with small issues.

An example (doesn't belong here except to make a psychology point) Mono vs poly cells is a big false issue. Monos are slightly more expensive, slightly more efficient but polys may be (at least according to one source) more efficient at higher temperature. Yet many people want mono because they "need" the best.

Assuming the glass is 1/4 inch or under, I doubt if the difference between solar or regular tempered is more than the 5% you measured. If from the price difference, you can make a panel 20% larger by using regular tempered, than it is a no-brainer. People arguing that the solar glass will pay for itself shortly don't realize that regular pays for itself faster.

Adding to my first post on reflection loss exceeding transmission loss is the further problem of shallow angle reflections.

If you keep your panel pointed at the sun, reflections are kept at 4% per air/glass surface. However, if at 45 degrees, only 70% as much sun hits the panel and still worse, reflections are worse than 4% per surface. Beyond 45 degrees, they get much worse. For this reason, Sanyo (my guess is other companies do as well) claims their rippled glass is superior. I am not so sure. My initial impulse was that it would be slightly inferior with the panel square to the sun, and slightly superior at angles. However, when the panel is angled, only some of the ripple (the sunny side) would be squarer to the sun. The tops of the ripple would be close to parallel to the back with similar reflections to flat glass and the far side of the ripple would be at an even shallower angle to the sun's rays.

I think there is shower glass that is essentially clear, rippled only on one side and probably tempered. I am going to add it to the growing list of experiments that need to be done.

However, as said above, it is a minor issue. Keeping the panel square to the sun may not be so minor, although other games are played, such as "rolling" solar cells around tubes so the sun always hits some squarely etc..

[MikeSolar]

Milosolar, it is good to know of alternatives, and that thin glass can be strong if tempered. My interest in 1/4 inch is that it tends to be code for certain doors and things and so is available used and cheap. Thinner is clearly lighter.

Longwolf, glad your meter readings confirmed my theory.

People seem to get obsessed with small issues.

An example (doesn't belong here except to make a psychology point) Mono vs poly cells is a big false issue. Monos are slightly more expensive, slightly more efficient but polys may be (at least according to one source) more efficient at higher temperature. Yet many people want mono because they "need" the best.

Assuming the glass is 1/4 inch or under, I doubt if the difference between solar or regular tempered is more than the 5% you measured. If from the price difference, you can make a panel 20% larger by using regular tempered, than it is a no-brainer. People arguing that the solar glass will pay for itself shortly don't realize that regular pays for itself faster.

Adding to my first post on reflection loss exceeding transmission loss is the further problem of shallow angle reflections.

If you keep your panel pointed at the sun, reflections are kept at 4% per air/glass surface. However, if at 45 degrees, only 70% as much sun hits the panel and still worse, reflections are worse than 4% per surface. Beyond 45 degrees, they get much worse. For this reason, Sanyo (my guess is other companies do as well) claims their rippled glass is superior. I am not so sure. My initial impulse was that it would be slightly inferior with the panel square to the sun, and slightly superior at angles. However, when the panel is angled, only some of the ripple (the sunny side) would be squarer to the sun. The tops of the ripple would be close to parallel to the back with similar reflections to flat glass and the far side of the ripple would be at an even shallower angle to the sun's rays.

I think there is shower glass that is essentially clear, rippled only on one side and probably tempered. I am going to add it to the growing list of experiments that need to be done.

However, as said above, it is a minor issue. Keeping the panel square to the sun may not be so minor, although other games are played, such as "rolling" solar cells around tubes so the sun always hits some squarely etc..

I am looking for an article I once had that said that the difference in cost between the two can be made up in 3-5 years of heat or power production. Of course this is for new glass and in a production environment but since you want used stuff all cost bets are off. The rippling on the inside is very different from the bathroom glass so there is no real comparison.

[cheapo]

wiki info

I started peaking into this and found several science sites and they all say about the same thing. Most of it is included on this wiki page.

Types of Picture Framing Glass

[edit]Regular (or "Clear")
Due to widespread availability and low cost, Soda Lime Glass is most commonly used for picture framing glass. Glass thicknesses typically range from thin 2.0 mm, to 2.5 mm. Clear glass has light transmission of ~ 90%, absorption of ~ 2%, and reflection of ~ 8%. Whereas absorption can be reduced by using low-iron glass, reflection can only be reduced by an anti-reflective surface treatment.
[edit]Low-iron (or "Extra-Clear," "Water White," etc)
Low-iron, or water white glass, is made using special iron free silica, and is generally only available in 2.0 mm thicknesses for picture framing applications. Because low iron glass light absorption can be as low as 0.5% vs. ~ 2% for clear glass, the light transmission will be significantly better vs. clear glass. Low iron glass has light transmission of ~91.5% and reflection of 8%.
[edit]Laminated Glass
Laminated glass offers shatter-resistance and protection from malicious breakage to art glazing. The most commonly used configuration is Glass + PVB Foil + Glass. Some variations of foils and glass thicknesses can offer shatter- and breakage-resistance or even bullet-resistance. The absorption of laminated glass depends on the glass substrates and foils used in the laminating process. Reflection of laminated glass is similar to monolithic glass, unless surface treatments are applied to reduce reflection.
[edit]Acrylic
Some types of acrylic glass can have the high light transmission and optical quality of glass. Acrylic is also light weight, compared to glass, and is shatter-resistant, making acrylic an attractive choice for framing large, over-sized works of art. In general, acrylic sheet scratches easily and retains a static charge, which can be problematic when framing pastels or charcoals. Some manufacturers, add dyes to acrylic glass to filter the UV light transmittance, and its surface can also be treated with both anti-static and anti-reflective coatings.[2]


Note their transmission figures include losses from both absorption and reflection, and I am assuming tempered "green" is similar to untempered "green" and tempered "low iron" is similar to untempered.

Also, their glass is 2 to 2.5mm, so assuming it is proportional, 1/4" is 6-7mm. So assume 3 times the absorption and the same reflectence.

So 1/4" low iron is about 100 minus 8% minus (3x0.5 or 1.5%) equals 90.5% transmission

and "Green" high iron is 100 minus 8% minus (3x2 or 6%) equals 86% transmission.

Such is consistent with longwolf's measurements.

Other sources claim acrylic is the best of the pile, slightly better than the finest optical (lens) glass. But the difference is marginal, and it still suffers from 8% reflection.. The difference in weight and thermal expansion (may crack cells) are more important.

As has been noted, 8% may be gained (a reduction of 2 air/glass surfaces) with encapsulation. If very thick glass, the difference becomes increasingly important, but here, with 1/8" to 1/4" glass, one can build a larger panel with more cells for the difference in money.

[MikeSolar]

I started peaking into this and found several science sites and they all say about the same thing. Most of it is included on this wiki page.

Types of Picture Framing Glass

[edit]Regular (or "Clear")
Due to widespread availability and low cost, Soda Lime Glass is most commonly used for picture framing glass. Glass thicknesses typically range from thin 2.0 mm, to 2.5 mm. Clear glass has light transmission of ~ 90%, absorption of ~ 2%, and reflection of ~ 8%. Whereas absorption can be reduced by using low-iron glass, reflection can only be reduced by an anti-reflective surface treatment.
[edit]Low-iron (or "Extra-Clear," "Water White," etc)
Low-iron, or water white glass, is made using special iron free silica, and is generally only available in 2.0 mm thicknesses for picture framing applications. Because low iron glass light absorption can be as low as 0.5% vs. ~ 2% for clear glass, the light transmission will be significantly better vs. clear glass. Low iron glass has light transmission of ~91.5% and reflection of 8%.
[edit]Laminated Glass
Laminated glass offers shatter-resistance and protection from malicious breakage to art glazing. The most commonly used configuration is Glass + PVB Foil + Glass. Some variations of foils and glass thicknesses can offer shatter- and breakage-resistance or even bullet-resistance. The absorption of laminated glass depends on the glass substrates and foils used in the laminating process. Reflection of laminated glass is similar to monolithic glass, unless surface treatments are applied to reduce reflection.
[edit]Acrylic
Some types of acrylic glass can have the high light transmission and optical quality of glass. Acrylic is also light weight, compared to glass, and is shatter-resistant, making acrylic an attractive choice for framing large, over-sized works of art. In general, acrylic sheet scratches easily and retains a static charge, which can be problematic when framing pastels or charcoals. Some manufacturers, add dyes to acrylic glass to filter the UV light transmittance, and its surface can also be treated with both anti-static and anti-reflective coatings.[2]


Note their transmission figures include losses from both absorption and reflection, and I am assuming tempered "green" is similar to untempered "green" and tempered "low iron" is similar to untempered.

Also, their glass is 2 to 2.5mm, so assuming it is proportional, 1/4" is 6-7mm. So assume 3 times the absorption and the same reflectence.

So 1/4" low iron is about 100 minus 8% minus (3x0.5 or 1.5%) equals 90.5% transmission

and "Green" high iron is 100 minus 8% minus (3x2 or 6%) equals 86% transmission.

Such is consistent with longwolf's measurements.

Other sources claim acrylic is the best of the pile, slightly better than the finest optical (lens) glass. But the difference is marginal, and it still suffers from 8% reflection.. The difference in weight and thermal expansion (may crack cells) are more important.

As has been noted, 8% may be gained (a reduction of 2 air/glass surfaces) with encapsulation. If very thick glass, the difference becomes increasingly important, but here, with 1/8" to 1/4" glass, one can build a larger panel with more cells for the difference in money.

Check out the website below:
I have used it for years of comparison of solar thermal products. The glass is the same regardless. 3.2mm and 4mm is standard.

http://www.spf.ch/Glasses.109.0.html?&L=6

Think about it....the cost difference may only be $30/sheet (for example) so how long does it take to make up $30 in electricity at your current rate? Low iron is used for a reason.

[inetdog]

Also, their glass is 2 to 2.5mm, so assuming it is proportional, 1/4" is 6-7mm. So assume 3 times the absorption and the same reflectence.

So 1/4" low iron is about 100 minus 8% minus (3x0.5 or 1.5%) equals 90.5% transmission

and "Green" high iron is 100 minus 8% minus (3x2 or 6%) equals 86% transmission.

Such is consistent with longwolf's measurements.

Other sources claim acrylic is the best of the pile, slightly better than the finest optical (lens) glass. But the difference is marginal, and it still suffers from 8% reflection.. The difference in weight and thermal expansion (may crack cells) are more important.

As has been noted, 8% may be gained (a reduction of 2 air/glass surfaces) with encapsulation. If very thick glass, the difference becomes increasingly important, but here, with 1/8" to 1/4" glass, one can build a larger panel with more cells for the difference in money.

Dead on!

When people choose glass for picture framing, they have to take into account that the light makes a round trip through the glass, so that the difference between 2% absorption and .5% absorption is more significant, although still small.

For clear, 90% x 90% = 81%, while for low iron, 91.5% x 91.5% is a whopping 83.7 % for a 3.3% improvement. (2.7 divided by 81 = 3.3% in case you are wondering where that number came from.) Possibly slight effects on color caused by different absorption at different frequencies are a more important concern where art is involved!

[MikeSolar]

Dead on!

When people choose glass for picture framing, they have to take into account that the light makes a round trip through the glass, so that the difference between 2% absorption and .5% absorption is more significant, although still small.

For clear, 90% x 90% = 81%, while for low iron, 91.5% x 91.5% is a whopping 83.7 % for a 3.3% improvement. (2.7 divided by 81 = 3.3% in case you are wondering where that number came from.) Possibly slight effects on color caused by different absorption at different frequencies are a more important concern where art is involved!

I'm not sure where your numbers are coming from. It is an old argument that low iron is around 92% and regular is 82-84% so it is roughly 9% better. Any absorption increases the temp in the glass and therefor the cell temp reducing the power output so try to minimize it.

If your typical system puts out 500kwh/m2 (and a power price of $.1/kwh) and you lose 9% by making a panel with regular glass, you are losing $5/m2 annually. Low iron MIGHT be $30 extra so it is a 6 year payback.

[cheapo]

perspective

Mike

We all need good links and I appreciate your glass supplier. Is your $30 extra for a 4x8 sheet? Do you make panels that big? If so, the incremental difference in solar glass may be worth it whether it is 5% or 9%. But such percentages must vary with thickness as surface reflection tends to equalize them.

I think the issue for more moderate sized panels is whether or not you can build a bigger panel with more cells for the money saved. At my expected $200 and 200 watt size (granted cheap used tempered glass) I might be able to get another 30 watts for $30 or 15%. (I think I am accounting for the extra cells, incremental extra sized glass, wire, encapsulant etc. From what I have seen, solar is usually more than $30 extra (new, compared to new tempered) for even moderate sizes. If space is a constraint, or if you can get a 4x8 sheet for only an extra $30, you may have an argument. My feeling is that everyone should have all information and they can pick their route.

[MikeSolar]

As the standards are 3.2 and 4mm glass, any numbers i see are based on those because you get the least expensive product with standardization. The $30 was just a example but long ago I went to a normal glass supplier and asked for 4x8' 4mm then went to the low iron guys and the cost was 60% more for low iron. At that time, there was only one supplier in North America where now there are many. My glass purchases were for thermal but the glass is basically the same.

Since you are talking PV, the standards are approx 1m by 1.65m for a 60 cell panel so you will get the best price if you stick to that range. I understand that is not your intent and I would like to see if you put an ROI on you calculation to see what is the better route, low iron or bigger panels with less transmission. So far, we are talking only first cost.

[longwolf]

Check out the website below:

Think about it....the cost difference may only be $30/sheet (for example) so how long does it take to make up $30 in electricity at your current rate? Low iron is used for a reason.

When I started this thread it cost me about $73 more to go with the low iron.
I'll stick with my argument, if you're building several panels anyway and have room for more, you could be better off using the normal tempered glass and using the saved money to make more panels.

It's nice to use the top of the line stuff, but it's not always the best choice.

Of course if you can get a better price on the low iron glass or you have a very limited amount of space for the panels, the story might be different.

You'll have to do the math for your own situation.

[bcroe]

Tempered Glass Price

I went to a local glass wholesaler, and bought 1/4" tempered glass for half
a dozen big panels, at $4.50 a square foot. And no shipping charge. The
hardware/remodeling places would sell it for several times that price, and
increasing turn around time while they get it from the same place. I didn't
go for low iron. Bought aluminum angle the same way.

[andy]

making laminated glass with emcapsulent

Having realized liquid encapsulent was not cost effective for pv panels I started using what I had to make laminated (also called bulletproof) glass. put the lower sheet on perfectly flat surface , just a few cc of encapsulent in the center and the other sheet on top with plenty of weight and watch the blob in the center spread out over the whole area. If glass is used for the backing surface of a panel , glass:EVA:cell:EVA:glass ,I think what you basically have is laminated glass , very strong. I think the way forward is 1mm glass on the face to minimize loss, and 3or4mm salvaged glass on the back, with price of cells quite reasonable now it doesn't make sense to spend a lot on glass: panels don't need to be excessively strong, once on the roof their safe, when did you last see hail the size of golf balls?